BACKGROUND: Sitting to supine fall in vital capacity (∆VC) is commonly used to screen for diaphragmatic dysfunction (DD), but the predictive threshold value varies. This systematic review aimed to compare the position-dependent change in vital capacity (VC) in patients with objectively confirmed DD. RESEARCH QUESTION: What is the optimal predictive value of ∆VC to diagnose DD. STUDY DESIGN AND METHODS: We searched Medline/PubMed, Embase and Scopus, including backward citations, for studies from database inception to December 5, 2023. Included trials measured position change in VC in adult patients with DD, confirmed independently by a parameter other than ∆VC. Risk of bias was assessed using the Downs and Black checklist. RESULTS: Of 497 records identified, 10 studies were included, totalling 393 adults, of which 284 had DD. In patients with confirmed unilateral diaphragmatic paralysis, mean change in VC ranged from 7 to 23%, and in those with bilateral diaphragmatic paralysis, from 19 to 37%. In studies providing only values for DD without specifying unilateral or bilateral involvement, it ranged from 31 to 42%. In control groups, it ranged from 3 to 9%. INTERPRETATION: The change in VC appears to be a valid test for confirming DD when using a cut-off value of 20%, though this approach results in very low sensitivity. A cut-off value of 15% should be used in a screening setting as an initial approach of a multimodal strategy, without being sensible enough to exclude milder forms of DD.